In making magnesium hydroxide from sea water or brines or other aqueous solutions containing magnesium ions, the magnesium ions are precipitated as magnesium hydroxide by treatment with alkali. Unfortunately certain impurities are also precipitated along with the magnesium hydroxide, a particularly undesirable species in certain circumstances being compounds of boron. In this connection, magnesium hydroxide used in the production of high purity refractory magnesium oxide is required to contain less than 0.10% B.sub.2 O.sub.3 /100 MgO and preferably less than 0.075% B.sub.2 O.sub.3 /100 MgO prior to the calcination stage, and is referred to as low boron magnesium hydroxide.
It is known that magnesium hydroxide containing less than 0.10% B.sub.2 O.sub.3 /100 MgO and preferably less than 0.075% B.sub.2 O.sub.3 /100 MgO can be precipitated from seawater by addition of alkali in excess of the stoichiometric requirement for precipitating 100% of the magnesium ions contained therein. Commonly an addition of alkali equivalent to 105-107% of the seawater magnesium ion content is made for this purpose. Unfortunately where the alkali employed is calcium based (as with lime or dolomitic lime), the high pH created by the excess hydroxyl ions present in solution due to the excess lime causes an increase in the amount of calcium ions adsorbed by the magnesium hydroxide, a large proportion of which cannot be removed by washing, and remains as an impurity when the magnesium hydroxide is calcined or deadburnt. The present invention provides a process for making low boron magnesium hydroxide without causing such excessive increase in the amount of calcium ions adsorbed.